Fusion of coccyx to sacrum in humans: prevalence, correlates, and effect on pelvic size, with obstetrical and evolutionary implications.

Humans do not have a tail, but we have four rudimentary coccygeal vertebrae. This study considers several issues pertaining to fusion of the coccyx to the sacrum, including prevalence, sexual differences, effect on pelvic size, and obstetrical and evolutionary implications. Previous research on sacral-coccygeal fusion has reported: (1) lower prevalence in females than males, (2) prevalence increases with age, (3) range in prevalence among 13 samples from 0 to 72%, and (4) obstetrical complications. This study uses a sample of 2,354 American skeletons of known sex, age 20 years and older to ascertain prevalence of sacral-coccygeal fusion and to evaluate some of its correlates. Results show that the sexes do not differ in prevalence of sacral-coccygeal fusion for five of seven decades of life, but that prevalence does increase with advancing age-from 24 to 47% from the third to eighth decades of life in females. Pelvimetric analysis of 132 females shows that those with sacral-coccygeal fusion have a shorter posterior sagittal diameter of the outlet compared to those without fusion; more than half of those with sacral-coccygeal fusion have an obstetrically contracted posterior sagittal diameter. Shortening of the posterior sagittal diameter is important, because its conjoint occurrence with a narrow subpubic arch may result in an obstetrically inadequate outlet. This study concludes that sacral-coccygeal fusion is a principal contributor to the evolution of sexual dimorphism in sacral angulation, which is a determinant of the length of the posterior sagittal diameter of the outlet.